Optical Frequency Domain Reflectometry (OFDR) is an effective system for measuring strain in optical fiber with high spatial resolution. See, e.g., U.S. Pat. Nos. 6,545,760; 6,566,648; 5,798,521; and 7,538,883. This high resolution measurement is useful in various industries by providing a near continuous measurement of strain along the fiber's length, e.g., see U.S. Pat. No. 8,714,026. Further, the high resolution strain measurement enables additional technologies such as optical fiber based shape and position sensing, e.g., see U.S. Pat. Nos. 7,772,541; 7,781,724; and 8,773,650.
In an OFDR measurement, a tunable laser is swept linearly in time across a range of frequencies. This sweep takes a fixed amount of time to perform. If the laser's sweep is linear in optical frequency vs. time, the sensing fiber remains stationary during the sweep duration, and the sensing fiber polarization mode dispersion or chromatic dispersion is negligible, each scattering location in the sensor produces a nearly pure sinusoidal interference signal. The frequency of this signal is proportional to the time-of-flight delay it takes for laser light to reach and return from the specific scattering location. Multiple scattering locations along the length of the fiber each add their own interference signals at their respective interference frequencies. In this way, OFDR produces a map of the sensing fiber in which interference frequency corresponds to time-of-flight delay between the OFDR instrument and each position along the length of the fiber.
OFDR measurements are susceptible to degradation of measurement coherence when the above assumptions are violated. In order for a single scattering location to produce a pure interference frequency, the fiber under test must remain static during the course of the laser's scan and interference data acquisition. Any disturbance that results in a change of total time-of-flight delay at the sensing location degrades the OFDR measurement. Such changes can be due to movement or rapid changes in the temperature or strain state of the fiber under test or of the fiber optic leads that connect the fiber under test to the instrument. Degradation of the coherence of the scatter pattern at a sensing element can lower the signal to noise ratio of the strain measurement for that sensing element.
Motion-induced degradation is a result of a scattering location's otherwise constant time-of-flight delay changing during the course of the laser sweep. Motion is typically represented by dynamic strain, the effects of which are cumulative (a detailed treatment is presented in U.S. Patent Application 20140336973). Any change in delay at a point in the sensor is the result of the accumulation of tension or compression along the entire length of fiber between the OFDR laser source and the sensing location. Therefore, if dynamic strain (induced by motion) is allowed to accumulate constructively over a long fiber length, the resulting degradation at the end of the fiber worsens with sensing fiber length. This dynamic strain scenario is encountered in many applications, such as a strain sensing fiber bonded to a vibrating beam.
Sophisticated processing techniques have been developed to mitigate or remove the effects of variations that occur during an OFDR measurement as described in U.S. Patent Application 20140336973. These techniques account for the cumulative effect of dynamic strain as measured at a selected point in the fiber. In this technique, a baseline measurement is collected a-priori, during which the fiber is held stationary; it is not exposed to any time-varying disturbance. In subsequent measurements, a computer-implemented algorithm compares a short region of perturbed measurement data to the corresponding region in the baseline reference. Subsequent data processing allows one to extract a measurement of the time-varying disturbance in the time-of-flight delay to that point along the length of the sensing fiber. This measurement is then used to form a correction for the OFDR measurement data. This technique greatly increases the range of OFDR applications allowing strain measurements to be performed in practical sensing environments in which ideal, static environments are not feasible.
Due to their reliance on pristine baseline data, these existing techniques are computationally expensive and may be difficult to apply efficiently in several practical applications. First, the comparison between perturbed measurement data and pristine baseline data must be performed over a finite length of optical fiber, e.g., on the order of 2 cm, that encompasses multiple scattering locations. If there is an optical frequency shift between the measurement and baseline states that is not constant along the comparison length, due to strain gradients and other time-constant phenomena that vary spatially along the length of the fiber, the measurement of the time variant time-of-flight delay will be in error, and the correction will not be effective. This finite-length requirement makes it difficult to employ existing techniques along the length of a sensor which exists in an unknown state.
Second, the known technique requires that the measurement and baseline data be properly aligned in time-of-flight delay before a comparative vibration measurement can be made; the chosen measurement region must be compared against the corresponding region in the reference data to determine and correct for any changes in optical delay to the sensor region used for vibration correction. In order to perform this alignment, the sensing algorithm must determine the cumulative state of the fiber at the sensing location. For long sensors, or in applications which require multiple vibration correction points, this becomes computationally expensive. Note that this task is made more difficult by the presence of vibration, which obscures attempts to find a valid starting point for the calculation. With large data sets obtained for longer sensing lengths, it may not be feasible to analyze the signals to determine the state of the fiber leading up to a location in the sensing fiber.
Third, because the existing technique is comparative (measurement data to baseline data), the perturbed measurement must not be changed so significantly that it no longer matches the pristine baseline well enough to deduce small changes in optical phase over the duration of the laser scan. This limits this technique's application under high levels of accumulated dynamic strain.
The inventors realized a need for technology that produces localized measurements of dynamic perturbations along the length of a sensing fiber with minimal or no reliance on a-priori baseline data and minimal or no need for information describing the state of the fiber leading up to that location of the fiber. A specialized sensing fiber is manufactured with discrete reflectors inscribed at known locations along the sensing optical fiber. Conventional wisdom in this field considers such a sensing system, including the specialized sensing fiber, as impractical. However, the inventors discovered that that such a sensing system could be made practically and greatly simplifies the process of measurement and subsequent correction of vibration, enabling more efficient application in a wide variety of situations, including those which present a processing challenge to the existing approach.
The discrete reference reflectors along the specialized sensing fiber allow initialization of the parameters associated with strain processing of OFDR data and measurement and correction for the effects of time-varying perturbations at or near a location of these discrete reference reflectors along the sensing fiber. Because the reference reflectors are inscribed specifically for this purpose, they are made to exhibit ideal or other known and distinguishing characteristics and therefore produce a high-fidelity measurement of dynamic strain with little computational load under large-amplitude motion, in the presence of spatial strain gradients, and/or over long measurement lengths. Hence, these discrete reference reflectors enable practical implementation of OFDR measurement techniques in the presence of time-varying disturbances in sensing fiber of unknown state, even in the case where these time-varying perturbations are allowed to accumulate over long lengths.
Further, a sensor manufactured with such discrete reference reflectors may be considered as a multi-point vibration sensor in which an independent vibration measurement is made at each reference reflector. Such a sensor also enables processing to allow the mitigation of other optical effects detrimental to OFDR measurements such as but not limited to polarization mode dispersion and chromatic dispersion. Additionally, the discrete reference reflectors are described to assist in identifying geometrical parameters of an optical sensing fiber such as but not limited to identification of physical distances along the length of the sensing fiber and cross sectional orientation of a multi-core optical fiber.
It is important to note that while the application of discrete localized reference reflectors is described for time-varying perturbations such as vibration, the technology described in this application applies to any phenomenon that undermines the fundamental OFDR assumption: that a single scattering location produces a pure frequency as a result of the OFDR swept-laser interferometry process. Because the laser's frequency must be swept linearly in time, this means that imperfect linearization, chromatic dispersion, and/or other optical frequency (spectral)-domain effects are registered locally and may be corrected for at or near each reference reflector, with minimal processing or a-priori knowledge required.
Example embodiments include an Optical Frequency Domain Reflectometry (OFDR) interrogation system and method for measuring a parameter of an optical sensing light guide that uses an optical interferometric interrogator. The optical sensing light guide is manufactured to have one or more localized reference reflectors. Each reference reflector produces a scattering event having a known scattering profile including elevated amplitude relative to scattering detected for neighboring segments of the optical sensing light guide. Each of the neighboring segments of the optical sensing light guide is a length of contiguous optical sensing light guide that is useable to initialize and perform a distributed OFDR sensing operation. Optical detection circuitry, coupled to the optical interferometric interrogator, detects optical interferometric measurement signals for a length of the optical sensing light guide. Data processing receives interferometric measurement signals from the optical detection circuitry and generates an interferometric measurement data set for the length of the optical sensing light guide. The interferometric measurement data set in proximity of each of the one or more reference reflectors is isolated, and an error signal is determined from the isolated data set for each of the reference reflectors. The error signal is provided for correction of the interferometric measurement data set and/or reported.
In some example embodiments, the segment is a contiguous length of sensing fiber ranging in length from several centimeters to tens or hundreds of meters and/or the segment does not include another reference reflector.
In some example embodiments, the known scattering profile is associated with an interference in a spectral domain of a discrete frequency proportional to a distance to a location of the scattering event along the length of the optical sensing light guide.
In some example embodiments, the reference reflector has a length that is on the order of a measurement resolution of the interferometric measurement data set.
In some example embodiments, the error signal is determined without requiring baseline OFDR measurement data for the optical sensing light guide.
In some example embodiments, the interferometric measurement data set is corrected locally for, at, or near each scattering event with little or no a-priori knowledge of the state of the optical sensing light guide up to that point in the fiber.
In some example embodiments, each scattering event has a spectral domain response of substantially constant magnitude and substantially increasing and substantially linear phase over a range of optical frequencies swept by a tunable laser source.
Each scattering event is distinguishable from Rayleigh scattering and/or scattering associated with fiber Bragg gratings (FBGs).
In some example embodiments, the error signal compensates the interferometric measurement data set for time-varying perturbations such as vibration effecting the optical sensing light guide and distorting the interferometric measurement data set of the parameter of the optical sensing light guide.
In some example embodiments, the error signal compensates the interferometric measurement data set for any phenomenon that undermines an assumption that a single scattering location produces a pure frequency as a result of an OFDR swept-laser interferometry process. The phenomenon may include non-linearity in time in an OFDR laser frequency sweep and/or, chromatic dispersion.
In some example embodiments, a region of the interferometric measurement data set is selected and isolated in a delay domain centered at a reference reflector. The isolated data is transformed to the spectral domain, and the phase argument is extracted from the spectral domain data to produce a measurement of the reflector's response. This is combined with the reflector's known response to produce a measured error signal.
In some example embodiments, an error signal is measured using a reference reflector. A portion of the interferometric measurement data set is selected and isolated in a delay domain to be compensated by a determined error signal. The isolated interferometric measurement set is transformed from the delay domain to a spectral domain, and the error signal is applied to the transformed isolated data. The compensated data is transformed to the delay domain to provide a compensated measurement data set for subsequent measurement of distributed sensor response.
In some example embodiments, an error signal is measured for each of multiple reference reflectors along the length of the optical fiber. The entire measurement data set in the delay domain is divided into a number of segments equal to the number of multiple reference reflectors. Each of the multiple measurement segments is compensated by an error signal from a reference reflector located in proximity to each of the portions of measurement data. A corrected interferometric measurement data set is produced by combining the data from each of the corrected measurement segments.
In some example embodiments, a error signal is measured using a reference reflector, and a derivative of the error signal is determined. A segment of the interferometric measurement data leading up to the location of the reference reflector is selected and an inverse Fourier transform performed. The spectral domain response of the interferometric measurement data portion is resampled to have an optical frequency increment proportional to the derivative of the error signal for each of the data points. A Fourier Transform performed on the resampled spectral domain response produces corrected interferometric measurement data that is corrected continuously over the interferometric measurement data portion.
Another aspect of the technology is an optical sensor including an optical fiber including multiple reference reflectors spaced along a length of the fiber. Each of the multiple reference reflectors producing a reference reflector having a known scattering profile including an elevated amplitude relative to scattering detected for neighboring segments of the optical fiber. Each of the segments is a length of contiguous fiber that is useable to initialize and perform a distributed Optical Frequency Domain Reflectometry (OFDR) sensing operation.
In some example embodiments, the optical sensor is a multi-point vibration sensor configured for vibration measurement and correction at each reference reflector.
In some example embodiments, the reference reflectors may be introduced in the optical fiber with a pulsed laser, included in a core of the optical fiber with minimal or no damage to the cladding and/or coating of the fiber, and/or spliced into the optical fiber.